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Qarot E, Guan Y, Hanani M. The protective barrier role of satellite glial cells in sensory ganglia. Glia 2024; 72:1054-1066. [PMID: 38450799 DOI: 10.1002/glia.24511] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2023] [Revised: 01/11/2024] [Accepted: 01/24/2024] [Indexed: 03/08/2024]
Abstract
Neurons in sensory ganglia are wrapped completely by satellite glial cells (SGCs). One putative function of SGCs is to regulate the neuronal microenvironment, but this role has received only little attention. In this study we investigated whether the SGC envelope serves a barrier function and how SGCs may control the neuronal microenvironment. We studied this question on short-term (<24 h) cell cultures of dorsal root ganglia and trigeminal ganglia from adult mice, which contain neurons surrounded with SGCs, and neurons that are not. Using calcium imaging, we measured neuronal responses to molecules with established actions on sensory neurons. We found that neurons surrounded by SGCs had a smaller response to molecules such as adenosine triphosphate (ATP), glutamate, GABA, and bradykinin than neurons without glial cover. When we inhibited the activity of NTPDases, which hydrolyze the ATP, and also when we inhibited the glutamate and GABA transporters on SGCs, this difference in the neuronal response was no longer observed. We conclude that the SGC envelope does not hinder diffusional passage, but acts as a metabolic barrier that regulates the neuronal microenvironment, and can protect the neurons and modulate their activity.
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Affiliation(s)
- Eman Qarot
- Laboratory of Experimental Surgery, Hadassah-Hebrew University Medical Center, Mount Scopus, Jerusalem, Israel
- Faculty of Medicine, Hebrew University of Jerusalem, Jerusalem, Israel
| | - Yun Guan
- Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA
- Department of Neurological Surgery, Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA
| | - Menachem Hanani
- Laboratory of Experimental Surgery, Hadassah-Hebrew University Medical Center, Mount Scopus, Jerusalem, Israel
- Faculty of Medicine, Hebrew University of Jerusalem, Jerusalem, Israel
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Braz JM, Hamel K, Craik V, Rodriguez-Rosado S, Bhardwaj K, Jewell M, Bieri G, Villeda SA, Basbaum AI. Pain and Itch Processing in Aged Mice. THE JOURNAL OF PAIN 2024; 25:53-63. [PMID: 37482234 DOI: 10.1016/j.jpain.2023.07.018] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/13/2023] [Revised: 06/30/2023] [Accepted: 07/18/2023] [Indexed: 07/25/2023]
Abstract
Most reports agree that aging negatively impacts pain processing and that the prevalence of chronic pain increases significantly with age. To improve current therapies, it is critical that aged animals be included in preclinical studies. Here we compared sensitivities to pain and itch-provoking stimuli in naïve and injured young and aged mice. Surprisingly, we found that in the absence of injury, aged male and female mice are significantly less responsive to mechanical stimuli and, in females, also to noxious thermal (heat) stimuli. In both older male and female mice, compared to younger (6-month-old mice), we also recorded reduced pruritogen-evoked scratching. On the other hand, after nerve injury, aged mice nevertheless developed significant mechanical hypersensitivity. Interestingly, however, and in contrast to young mice, aged mice developed both ipsilateral and contralateral postinjury mechanical allodynia. In a parallel immunohistochemical analysis of microglial and astrocyte markers, we found that the ipsilateral to the contralateral ratio of nerve injury-induced expression decreased with age. That observation is consistent with our finding of contralateral hypersensitivity after nerve injury in the aged but not the young mice. We conclude that aging has opposite effects on baseline versus postinjury pain and itch processing. PERSPECTIVE: Aged male and female mice (22-24 months) are less sensitive to mechanical, thermal (heat), and itch-provoking stimuli than are younger mice (6 months).
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Affiliation(s)
- João M Braz
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Katherine Hamel
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Veronica Craik
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Sian Rodriguez-Rosado
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Karnika Bhardwaj
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Madison Jewell
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Gregor Bieri
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Saul A Villeda
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Allan I Basbaum
- Department of Anatomy, University of California, San Francisco, San Francisco, California
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Morishita M, Kobayashi K, Mitsuzuka M, Takagi R, Ono K, Momma R, Tsuneoka Y, Horio S, Tsukahara S. Two-Step Actions of Testicular Androgens in the Organization of a Male-Specific Neural Pathway from the Medial Preoptic Area to the Ventral Tegmental Area for Modulating Sexually Motivated Behavior. J Neurosci 2023; 43:7322-7336. [PMID: 37722849 PMCID: PMC10621776 DOI: 10.1523/jneurosci.0361-23.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2023] [Revised: 08/16/2023] [Accepted: 09/13/2023] [Indexed: 09/20/2023] Open
Abstract
The medial preoptic area (MPOA) is a sexually dimorphic region of the brain that regulates social behaviors. The sexually dimorphic nucleus (SDN) of the MPOA has been studied to understand sexual dimorphism, although the anatomy and physiology of the SDN is not fully understood. Here, we characterized SDN neurons that contribute to sexual dimorphism and investigated the mechanisms underlying the emergence of such neurons and their roles in social behaviors. A target-specific neuroanatomical study using transgenic mice expressing Cre recombinase under the control of Calb1, a gene expressed abundantly in the SDN, revealed that SDN neurons are divided into two subpopulations, GABA neurons projecting to the ventral tegmental area (VTA), where they link to the dopamine system (CalbVTA neurons), and GABA neurons that extend axons in the MPOA or project to neighboring regions (CalbnonVTA neurons). CalbVTA neurons were abundant in males, but were scarce or absent in females. There was no difference in the number of CalbnonVTA neurons between sexes. Additionally, we found that emergence of CalbVTA neurons requires two testicular androgen actions that occur first in the postnatal period and second in the peripubertal period. Chemogenetic analyses of CalbVTA neurons indicated a role in modulating sexual motivation in males. Knockdown of Calb1 in the MPOA reduced the intromission required for males to complete copulation. These findings provide strong evidence that a male-specific neural pathway from the MPOA to the VTA is organized by the two-step actions of testicular androgens for the modulation of sexually motivated behavior.SIGNIFICANCE STATEMENT The MPOA is a sexually dimorphic region of the brain that regulates social behaviors, although its sexual dimorphism is not fully understood. Here, we describe a population of MPOA neurons that contribute to the sexual dimorphism. These neurons only exist in masculinized brains, and they project their axons to the ventral tegmental area, where they link to the dopamine system. Emergence of such neurons requires two testicular androgen actions that occur first in the postnatal period and second in the peripubertal period. These MPOA neurons endow masculinized brains with a neural pathway from the MPOA to the ventral tegmental area and modulate sexually motivated behavior in males.
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Affiliation(s)
- Masahiro Morishita
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Kaito Kobayashi
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Moeri Mitsuzuka
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Ryo Takagi
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Kota Ono
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Rami Momma
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
| | - Yousuke Tsuneoka
- Department of Anatomy, Faculty of Medicine, Toho University, Tokyo 43-8540, Japan
| | - Shuhei Horio
- Division of Endocrinology and Metabolism, National Institute for Physiological Sciences, Okazaki 444-8585, Japan
| | - Shinji Tsukahara
- Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan
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Ward PJ, Clanton SL, English AW. Optogenetically enhanced axon regeneration: motor versus sensory neuron-specific stimulation. Eur J Neurosci 2018; 47:294-304. [PMID: 29363200 DOI: 10.1111/ejn.13836] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2017] [Revised: 01/05/2018] [Accepted: 01/11/2018] [Indexed: 12/25/2022]
Abstract
Brief neuronal activation in injured peripheral nerves is both necessary and sufficient to enhance motor axon regeneration, and this effect is specific to the activated motoneurons. It is less clear whether sensory neurons respond in a similar manner to neuronal activation following peripheral axotomy. Further, it is unknown to what extent enhancement of axon regeneration with increased neuronal activity relies on a reflexive interaction within the spinal circuitry. We used mouse genetics and optical tools to evaluate the precision and selectivity of system-specific neuronal activation to enhance axon regeneration in a mixed nerve. We evaluated sensory and motor axon regeneration in two different mouse models expressing the light-sensitive cation channel, channelrhodopsin (ChR2). We selectively activated either sensory or motor axons using light stimulation combined with transection and repair of the sciatic nerve. Regardless of genotype, the number of ChR2-positive neurons whose axons had regenerated successfully was greater following system-specific optical treatment, with no effect on the number of ChR2-negative neurons (whether motor or sensory neurons). We conclude that acute system-specific neuronal activation is sufficient to enhance both motor and sensory axon regeneration. This regeneration-enhancing effect is likely cell autonomous.
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Affiliation(s)
- Patricia J Ward
- Department of Cell Biology, Emory University School of Medicine, 615 Michael Street, Room 425, Atlanta, GA, 30322, USA
| | - Scott L Clanton
- Department of Cell Biology, Emory University School of Medicine, 615 Michael Street, Room 425, Atlanta, GA, 30322, USA
| | - Arthur W English
- Department of Cell Biology, Emory University School of Medicine, 615 Michael Street, Room 425, Atlanta, GA, 30322, USA
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Tran M, Kuhn JA, Bráz JM, Basbaum AI. Neuronal aromatase expression in pain processing regions of the medullary and spinal cord dorsal horn. J Comp Neurol 2017. [PMID: 28649695 DOI: 10.1002/cne.24269] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
In both acute and chronic pain conditions, women tend to be more sensitive than men. This sex difference may be regulated by estrogens, such as estradiol, that are synthesized in the spinal cord and brainstem and act locally to influence pain processing. To identify a potential cellular source of local estrogen, here we examined the expression of aromatase, the enzyme that catalyzes the conversion of testosterone to estradiol. Our studies focused on primary afferent neurons and on their central targets in the spinal cord and medulla as well as in the nucleus of the solitary tract, the target of nodose ganglion-derived visceral afferents. Immunohistochemical staining in an aromatase reporter mouse revealed that many neurons in laminae I and V of the spinal cord dorsal horn and caudal spinal trigeminal nucleus and in the nucleus of the solitary tract express aromatase. The great majority of these cells also express inhibitory interneuron markers. We did not find sex differences in aromatase expression and neither the pattern nor the number of neurons changed in a sciatic nerve transection model of neuropathic pain or in the Complete Freund's adjuvant model of inflammatory pain. A few aromatase neurons express Fos after cheek injection of capsaicin, formalin, or chloroquine. In total, given their location, these aromatase neurons are poised to engage nociceptive circuits, whether it is through local estrogen synthesis or inhibitory neurotransmitter release.
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Affiliation(s)
- May Tran
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Julia A Kuhn
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - João M Bráz
- Department of Anatomy, University of California, San Francisco, San Francisco, California
| | - Allan I Basbaum
- Department of Anatomy, University of California, San Francisco, San Francisco, California
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Bráz JM, Wang F, Basbaum AI. Presynaptic Inputs to Any CNS Projection Neuron Identified by Dual Recombinant Virus Infection. PLoS One 2015; 10:e0140681. [PMID: 26470056 PMCID: PMC4607402 DOI: 10.1371/journal.pone.0140681] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2015] [Accepted: 09/29/2015] [Indexed: 12/24/2022] Open
Abstract
Although neuroanatomical tracing studies have defined the origin and targets of major projection neurons (PN) of the central nervous system (CNS), there is much less information about the circuits that influence these neurons. Recently, genetic approaches that use Cre recombinase-dependent viral vectors have greatly facilitated such circuit analysis, but these tracing approaches are limited by the availability of Cre-expressing mouse lines and the difficulty in restricting Cre expression to discrete regions of the CNS. Here, we illustrate an alternative approach to drive Cre expression specifically in defined subsets of CNS projection neurons, so as to map both direct and indirect presynaptic inputs to these cells. The method involves a combination of Cre-dependent transneuronal viral tracers that can be used in the adult and that does not require genetically modified mice. To trigger Cre-expression we inject a Cre-expressing adenovirus that is retrogradely transported to the projection neurons of interest. The region containing the retrogradely labeled projection neurons is next injected with Cre-dependent pseudorabies or rabies vectors, which results in labeling of poly- and monosynaptic neuronal inputs, respectively. In proof-of-concept experiments, we used this novel tracing system to study the circuits that engage projection neurons of the superficial dorsal horn of the spinal cord and trigeminal nucleus caudalis, neurons of the parabrachial nucleus of the dorsolateral pons that project to the amygdala and cortically-projecting neurons of the lateral geniculate nucleus. Importantly, because this dual viral tracing method does not require genetically derived Cre-expressing mouse lines, inputs to almost any projection system can be studied and the analysis can be performed in larger animals, such as the rat.
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Affiliation(s)
- João M. Bráz
- Department of Anatomy, University of California San Francisco, San Francisco, CA, 94143, United States of America
| | - Fan Wang
- Department of Cell Biology, Duke University, Durham, NC, 27710, United States of America
| | - Allan I. Basbaum
- Department of Anatomy, University of California San Francisco, San Francisco, CA, 94143, United States of America
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Zhang Y, Zhao S, Rodriguez E, Takatoh J, Han BX, Zhou X, Wang F. Identifying local and descending inputs for primary sensory neurons. J Clin Invest 2015; 125:3782-94. [PMID: 26426077 DOI: 10.1172/jci81156] [Citation(s) in RCA: 80] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2015] [Accepted: 07/23/2015] [Indexed: 12/25/2022] Open
Abstract
Primary pain and touch sensory neurons not only detect internal and external sensory stimuli, but also receive inputs from other neurons. However, the neuronal derived inputs for primary neurons have not been systematically identified. Using a monosynaptic rabies viruses-based transneuronal tracing method combined with sensory-specific Cre-drivers, we found that sensory neurons receive intraganglion, intraspinal, and supraspinal inputs, the latter of which are mainly derived from the rostroventral medulla (RVM). The viral-traced central neurons were largely inhibitory but also consisted of some glutamatergic neurons in the spinal cord and serotonergic neurons in the RVM. The majority of RVM-derived descending inputs were dual GABAergic and enkephalinergic (opioidergic). These inputs projected through the dorsolateral funiculus and primarily innervated layers I, II, and V of the dorsal horn, where pain-sensory afferents terminate. Silencing or activation of the dual GABA/enkephalinergic RVM neurons in adult animals substantially increased or decreased behavioral sensitivity, respectively, to heat and mechanical stimuli. These results are consistent with the fact that both GABA and enkephalin can exert presynaptic inhibition of the sensory afferents. Taken together, this work provides a systematic view of and a set of tools for examining peri- and extrasynaptic regulations of pain-afferent transmission.
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Primary afferent and spinal cord expression of gastrin-releasing peptide: message, protein, and antibody concerns. J Neurosci 2015; 35:648-57. [PMID: 25589759 DOI: 10.1523/jneurosci.2955-14.2015] [Citation(s) in RCA: 81] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
There is continuing controversy relating to the primary afferent neurotransmitter that conveys itch signals to the spinal cord. Here, we investigated the DRG and spinal cord expression of the putative primary afferent-derived "itch" neurotransmitter, gastrin-releasing peptide (GRP). Using ISH, qPCR, and immunohistochemistry, we conclude that GRP is expressed abundantly in spinal cord, but not in DRG neurons. Titration of the most commonly used GRP antiserum in tissues from wild-type and GRP mutant mice indicates that the antiserum is only selective for GRP at high dilutions. Paralleling these observations, we found that a GRPeGFP transgenic reporter mouse has abundant expression in superficial dorsal horn neurons, but not in the DRG. In contrast to previous studies, neither dorsal rhizotomy nor an intrathecal injection of capsaicin, which completely eliminated spinal cord TRPV1-immunoreactive terminals, altered dorsal horn GRP immunoreactivity. Unexpectedly, however, peripheral nerve injury induced significant GRP expression in a heterogeneous population of DRG neurons. Finally, dual labeling and retrograde tracing studies showed that GRP-expressing neurons of the superficial dorsal horn are predominantly interneurons, that a small number coexpress protein kinase C gamma (PKCγ), but that none coexpress the GRP receptor (GRPR). Our studies support the view that pruritogens engage spinal cord "itch" circuits via excitatory superficial dorsal horn interneurons that express GRP and that likely target GRPR-expressing interneurons. The fact that peripheral nerve injury induced de novo GRP expression in DRG neurons points to a novel contribution of this peptide to pruritoceptive processing in neuropathic itch conditions.
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Su J, Gao T, Shi T, Xiang Q, Xu X, Wiesenfeld-Hallin Z, Hökfelt T, Svensson CI. Phenotypic changes in dorsal root ganglion and spinal cord in the collagen antibody-induced arthritis mouse model. J Comp Neurol 2015; 523:1505-28. [DOI: 10.1002/cne.23749] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2014] [Revised: 12/26/2014] [Accepted: 01/24/2015] [Indexed: 12/31/2022]
Affiliation(s)
- Jie Su
- Department of Physiology and Pharmacology; Karolinska Institutet; Stockholm 171 77 Sweden
| | - Tianle Gao
- Department of Physiology and Pharmacology; Karolinska Institutet; Stockholm 171 77 Sweden
| | - Tiejun Shi
- Department of Neuroscience; Karolinska Institutet; Stockholm 171 77 Sweden
| | - Qiong Xiang
- Department of Neuroscience; Karolinska Institutet; Stockholm 171 77 Sweden
| | - Xiaojun Xu
- Department of Physiology and Pharmacology; Karolinska Institutet; Stockholm 171 77 Sweden
| | | | - Tomas Hökfelt
- Department of Neuroscience; Karolinska Institutet; Stockholm 171 77 Sweden
| | - Camilla I. Svensson
- Department of Physiology and Pharmacology; Karolinska Institutet; Stockholm 171 77 Sweden
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VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review. ISRN NEUROLOGY 2013; 2013:829753. [PMID: 24349795 PMCID: PMC3856137 DOI: 10.1155/2013/829753] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/02/2013] [Accepted: 08/25/2013] [Indexed: 02/07/2023]
Abstract
Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.
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Local and remote immune-mediated inflammation after mild peripheral nerve compression in rats. J Neuropathol Exp Neurol 2013; 72:662-80. [PMID: 23771220 DOI: 10.1097/nen.0b013e318298de5b] [Citation(s) in RCA: 69] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
After experimental nerve injuries that extensively disrupt axons, such as chronic constriction injury, immune cells invade the nerve, related dorsal root ganglia (DRGs), and spinal cord, leading to hyperexcitability, raised sensitivity, and pain. Entrapment neuropathies, such as carpal tunnel syndrome, involve minimal axon damage, but patients often report widespread symptoms. To understand the underlying pathology, a tube was placed around the sciatic nerve in 8-week-old rats, leading to progressive mild compression as the animals grew. Immunofluorescence was used to examine myelin and axonal integrity, glia, macrophages, and T lymphocytes in the nerve, L5 DRGs, and spinal cord after 12 weeks. Tubes that did not constrict the nerve when applied caused extensive and ongoing loss of myelin, together with compromise of small-, but not large-, diameter axons. Macrophages and T lymphocytes infiltrated the nerve and DRGs. Activated glia proliferated in DRGs but not in spinal cord. Histologic findings were supported by clinical hyperalgesia to blunt pressure and cold allodynia. Tubes that did not compress the nerve induced only minor local inflammation. Thus, progressive mild nerve compression resulted in chronic local and remote immune-mediated inflammation depending on the degree of compression. Such neuroinflammation may explain the widespread symptoms in patients with entrapment neuropathies.
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Untypical connectivity from olfactory sensory neurons expressing OR37 into higher brain centers visualized by genetic tracing. Histochem Cell Biol 2012; 137:615-28. [PMID: 22294261 DOI: 10.1007/s00418-012-0919-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 01/13/2012] [Indexed: 10/14/2022]
Abstract
The OR37 subfamily of odorant receptors(ORs) exists exclusively in mammals. In contrast to ORs in general, they are highly conserved within and across species.These unique features raise the question, whether olfactory information gathered by the OR37 sensory cells is processed in specially designated brain areas. To elucidate the wiring of projection neurons from OR37 glomeruli into higher brain areas, tracing experiments were performed.The application of DiI onto the ventral area of the olfactory bulb, which harbors the OR37 glomeruli, led to the labeling of fibers not only in the typical olfactory cortical regions,but also in the medial amygdala and the hypothalamus. To visualize the projections from a defined OR37 glomerulus more precisely, transgenic mice were studied in which olfactory sensory neurons co-express the receptor subtype OR37C and the transsynaptic tracer wheat germ agglutinin(WGA). WGA became visible not only in the OR37C sensory neurons and the corresponding OR37C glomerulus,but also in cell somata located in the mitral/tufted cell layer adjacent to the OR37C glomerulus, indicating a transfer of WGA onto projection neurons. In the brain, WGA immunoreactivity was not detectable in typical olfactory cortical areas, but instead in distinct areas of the medial amygdala.Detailed mapping revealed that the WGA immunoreactivity was restricted to the posterior-dorsal subnucleus of the medial amygdala. In addition, WGA immunoreactivity was visible in some well-circumscribed areas of the hypothalamus.These results are indicative for a unique connectivity from OR37C sensory cells into higher brain centers.
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